Historically, maize (corn) has been used as a source of food for human and animal consumption. Today, maize supplies about twenty percent of the world's calories. Any environmental stress factor that affects maize can have an impact on maize availability. Thus reduction of the sensitivity of maize to GLS is understandably of importance. Gray leaf spot can result in yield loss as great as 20%.
Gray leaf spot has gained prominence the last decade. It has the potential to become a significant problem, not only in the mid-Atlantic region of the U.S. but in other major corn producing areas as well. GLS has been reported in Missouri, Iowa and Nebraska. This dramatic increase of locations having the disease is associated with no-tillage or reduced-tillage production methods. These conditions contribute to overwintering of the fungus and early infection the following season.
The fungal pathogen Cercospora zeae-maydis which causes GLS, characteristically produces long, rectangular, grayish-tan leaf lesions which run parallel to the leaf veins. These lesions may blight the entire leaf. Blighting due to GLS is associated with the premature loss of photosynthetic area. The dominant sink of the post-flowering maize plant is the ear. Blighting induces the plant to transfer photosynthate from the stalk and roots to the ear at high levels causing premature senescence.
The level of resistance to GLS in commercial hybrids and inbreds has been evaluated. Resistant or tolerant genotypes have been reported but few, if any, can be classified highly resistant. Although there was little evidence of any strong resistance to GLS in the commercially available hybrids, potential sources of resistance in several inbred lines (Va59 and Pa887p) have been reported. The Pennsylvania State University Experiment Station has released an inbred, Pa875, with resistance to GLS which appears to be multigenic and additive in nature. This inbred is a good source of resistant material, but it does not solve the GLS problem. There is a need for commercially acceptable hybrids which are GLS resistant. And there is a remaining need for an easy and trackable method of development of other resistant maize inbreds and hybrids.
In an article by D. M. Bubeck, M. M. Goodman, W. D. Beavis and D. Grant, 1993, entitled Quantitative Trait Loci Controlling Resistance to Gray Leaf Spot in Maize, Crop Sci.33:838-847, the authors presented one loci for GLS which was consistent and numerous loci for GLS which were inconsistent. On the other hand, the present invention teaches four consistent loci for GLS. Thus, the present invention is not specifically taught. This article attempted to identify quantitative trait loci (QTL) on the basis of marker associations with GLS means over all ratings taken in environments. The paper indicates the use of RFLP mapping of two different donors having partial resistance to GLS (ADENT and NC250A) in three populations. The reference shows that the QTL were inconsistent over environments. Although this paper leads to the conclusion that the practical use of markers is limited because of environmental influence which gives inconsistent results; the paper recites a breeding application. The paper indicates selection should be based on the phenotype and the genotype of the inbred. It should be noted that Table 8 in the paper indicates that the marker association would have led to the selection of nine families which rated poorly for GLS in a visual rating.
The paper indicates a number of identified GLS loci which seem to vary by environment. The present invention, on the other hand, clearly identifies loci of Va14 which remains consistent throughout environments. Additionally, the present invention teaches a method of improving GLS susceptible inbred lines by selecting for the genotype identified by markers of the desired targeted inbred and selecting for the introgression of the Va14 material at the selected loci. This invention clearly covers an inbred that has targeted inbred material in substantially all chromosomal regions except at least one or more of those identified as loci 1-4. In this invention at least some of the loci 1-4 in the chromosomal regions contain the introgressed GLS resistant material. This invention is not taught by the above identified reference.
Unfortunately, due to the genetics of GLS resistant maize, it is difficult to transfer this resistance to new inbreds. And it is difficult to transfer it to new hybrid products. In fact, it is quite common to see some poor agronomic traits and loss of resistance associated with moving the resistance trait from inbred to inbred.
Heretofore, few if any, truly agronomically desirable varieties of corn have resistance to GLS. This invention discovered that four chromosomal regions control the maize plant's response to the GLS in Va14. A progeny containing these genes, two of which are recessive in nature, within its genome is expected to be a rare occurrence.
Maize breeding combines two inbreds to produce a hybrid having a desired mix of traits. Getting the correct mix of traits from two inbreds in a hybrid can be difficult, especially when traits are not directly associated with phenotypic characteristics.
In a conventional breeding program, pedigree breeding and recurrent selection breeding methods are employed to develop new inbred lines with desired traits. Maize breeding programs attempt to develop these inbred lines by self-pollinating plants and selecting the desirable plants from the populations. Inbreds tend to have poor vigor and low yield; however, the progeny of an inbred cross usually evidences vigor. The progeny of a cross between two inbreds is often identified as an F.sub.1 hybrid. In traditional breeding F.sub.1 hybrids are evaluated to determine whether they show agronomically important and desirable traits. Identification of desirable agronomic traits has typically been done by breeders' expertise. A plant breeder identifies a desired trait for the area in which his plants are to be grown and selects inbreds which appear to pass the desirable trait or traits on to the hybrid. Conventional plant breeders rely on phenotypic traits of the inbreds for selection purposes.
Modern plant breeding technology looks at the genotypic material (chromosomes) for plant breeding purposes. One method of looking at plant genotypes is to use Restriction Fragment Length Polymorphisms (RFLPs). RFLPs can be used to identify the chromosomal regions which affect the agronomic traits in the plant genome. The plant breeder can use this information to introgress the trait into the inbred line for ultimate expression in the hybrid.
Maize is a ten chromosome plant. Each chromosome has a short arm with a distal and proximal end and a long arm having a distal and proximal end. The short arm proximal end and long arm proximal end define the edges of the centromere. Each chromosome is made up of strands of the deoxyribonucleic acid (DNA) molecule which has a specific nucleic acid sequence. Selected restriction endonucleases will identify a specific base sequence and cleave the DNA molecule wherever this sequence occurs. The resultant cleaved portions are called restriction fragments. These restriction fragments can be separated by size by electrophoresis through agarose gels.
The DNA of two individual maize plants will differ in sequence at a variety of sites. Because of this difference, restriction endonucleases may cleave the two plants' DNA at a different sites. A polymorphism in the length of restriction fragments is produced when the fragments of the two plants have different lengths. A polymorphism is detected by placing the fragments on an agarose gel electrophoresis and allowing them to separate by size over distance. A Southern blot is then used. The fragments of the DNA are physically transferred on to a membrane, then nucleic acid hybridization detects the sequences by hybridization of the single strand of DNA (probe) on the Southern blot. The nucleic acid reforms double stranded DNA. A probe is used to detect a particular (DNA) sequence. One detection method uses autoradiography.
A variety of maize genes have been mapped and identified using RFLPs. Certain molecular markers are used to identify chromosomal areas associated with certain traits. A large number of molecular markers including RFLPs have been applied to the maize genome. A detailed maize genetic map has been constructed.
A variety of traits has been identified by RFLPs; for example, pericarp color has been linked to UMC185(P1)on the short arm of chromosome one of the maize plant. Probes BNL6.29 and UMC85 on chromosome six of the maize plant have been identified with Maize Dwarf Mosaic Virus (MDMV) strain A resistance in maize. Likewise, a variety of other traits have been genetically identified and placed on the maize genetic linkage map.
It is not easy to recognize the desired chromosomal location of a desired trait. Although RFLPs are a tool which can be employed to help identify the chromosomal region to which the trait appears to be linked, RFLPs are not a solution in and of themselves. RFLPs are simply a tool of identification. It should be noted that all of these specific chromosomal regions associated with GLS resistance have not been previously mapped or identified using probes.
Little has hitherto been known about the genes responsible for resistance to GLS, except that GLS resistance seemed to be additive in nature and difficult to carry into new plants. The present invention allows selection of progeny which contain the genomic background of the agronomically desirable parent and the genomic trait of the GLS donor parent. There is a need for the identification of these specific locations of genes associated with resistance to GLS to permit their tracking. Tracked material can be introgressed into new plants through traditional breeding. There also remains a need for a method of transferring resistance to GLS to corn inbreds having desirable agronomic traits and adapted for various regions where GLS is found. There remains a need for resistant GLS inbreds and hybrids.